This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present invention relates to a valve assembly, in particular to a valve assembly in which fluid pressure across the valve is required to be equalized before the position of the valve is changed, such as a gate valve or a ball valve. The valve assembly of the present invention finds particular use in wellhead assemblies and the control of fluids produced from subterranean wells, in particular in subsea locations.
Valves of differing designs and operating principles are also known. For example gate valves and ball valves are known. These forms of valve are known for use in controlling fluid flow and are generally operated between a fully open position and a fully closed position, as fluid flow is required. They offer some ability to control the flow of fluid between the fully open flowrate and zero flow achieved with the valve in the fully closed position. However, they are generally only used at relatively low pressures. Gate valves and ball valves are generally only used where no significant pressure differential exists between the fluid upstream and downstream of the valve. An alternative form of valve is the butterfly valve, generally also operated in a fully open or fully closed position, to control fluid flow. As butterfly valves are generally efficient only at low pressures, they are seldom used in wellhead installations, where a valve must be able to cope with being exposed to fluid at full wellhead pressure.
Check valves are used to allow fluid flow in a given direction, typically once a predetermined threshold pressure has been achieved and the valve is ‘cracked’, but to prevent the flow of fluids in the reverse direction. Various designs of check valve are known and operated, including ball check valves, diaphragm check valves and swing check valves.
Valves, such as gate valves and ball valves, are generally used in situations where the fluid pressure upstream of the valve is generally the same as the fluid pressure downstream of the valve, as noted above. In use, when changing the position of the valve, for example when closing an open valve or opening a closed valve, it is necessary to equalize the pressure of fluid on the upstream and downstream sides of the valve. Conventional valve designs do not in general provide for the pressure to be equalized in this manner without movement of the valve itself.
There is a need for an improved valve assembly, in particular for use in the control of fluids produced from a subterranean well, for example in a wellhead assembly.